Known heat pump systems execute a hot water supply operation using an air-side heat exchanger as an evaporator when the outside air temperature is higher than the geothermal-side temperature, and execute a hot water supply operation using a geothermal-side heat exchanger as an evaporator when the outside air temperature is lower than the geothermal-side temperature (see, for example, Patent Literature 1).
There have also been air-conditioning systems which cause a refrigerant to flow to an air-side heat exchanger when the temperature of the refrigerant is higher than a predetermined temperature and which cause a refrigerant to flow to a heat exchanger utilizing earth heat (geothermal-side heat exchanger) when the temperature of the refrigerant is lower than or equal to the predetermined temperature (see, for example, Patent Literature 2).